In recent years, the fast growing of datacom networks and the large and ever-increasing amount of services made available to the users over such networks have led to a remarkable growth of traffic which is heavily affecting network performance. Terrestrial broadband wireless access (BWA), point-to-multipoint (PMP) systems with dynamic capacity allocation are thus expected to support and provide an increasing number of applications and services, including the highly demanding multimedia and last generation mobile network infrastructure. The former ones include, for instance, fast Internet video and video on demand (VoD), the latter ones include the 2G/3G mobile network infrastructure, particularly as concerns connection among mobile base stations and switch sites.
In a typical system, a PMP system topology scheme comprises a plurality of base stations connecting several access terminals through a particular media and by using a particular multiple access scheme.
For illustrative purposes and clarity reasons only, the topology scheme is described for a single base station. The base station is preferably a radio node station (RNS), i.e. a site interfacing one or more radio access terminals through a radio connection. A radio node station comprises at least one radio unit with antenna and modem to make it possible for radio access terminals to access services provided by the network to which the radio Base Station is connected.
A radio access terminal (RAT) is a terminal device used to connect end users or mobile base stations to the network via a radio connection towards a radio node station.
An important issue is the cost of RATs, which mainly affects the cost per link. The cost of a radio node station, is spread over a large amount of RATs and, for this reasons, is of less significance.
In full-duplex (FDD) terminals, a first radio frequency band is used for downstream traffic from an RNS to the RAT and a second radio frequency band is used for upstream traffic from the RAT to the RNS. Moreover, a guard frequency band separates the upstream and downstream bands. This allows data transfer in both directions at the same time but, as a drawback, implies the use of expensive equipment, which may decrease the appeal of wireless systems and services. An approach which can be used in order to decrease costs is to employ the half duplex mechanism (H-FDD). In a half duplex system, different frequencies are still used, but the transmitter and the receiver of a RAT cannot operate at the same time. Half-duplex RATs are less complex and cheaper than their full-duplex counterparts, since specialised technical components, like diplexer filters, are no longer needed, and a single multi-chip module (MCM) can be used for both receiving and transmitting activities.
In BWA/PMP systems, as in most communication networks, information is transferred in frames. A frame is a block of data containing both actual data and ancillary information to synchronise transmission and to supply transmission data to the RATs. BWA/PMP frame structures can dynamically support both FDD and H-FDD terminals and use data structures based on two alternative solutions, namely slot based structures and frame based structures. However, H-FDD terminals have a considerable impact on the access system. In fact, as this kind of terminal cannot receive and transmit at the same time, performance may be affected.
A typical frame structure which is used to handle both full-duplex and half-duplex terminals is provided by the ETSI BRAN HIPERACCESS standard, which proposes a downlink frame based structure comprising a preamble, a downlink map, an uplink map, a first downlink portion based on TDM and a second downlink portion based on TDMA (time division multiple access). The size or duration of the sections is variable and the second portion is optional. Broadly speaking, the two maps at the beginning of the frame contain the so called broadcast channel, that is information regarding the downlink and uplink frame structures. It is to be noted that, as a convention, in a point to multipoint wireless broadband access system (PMP/WBA) the term “uplink” identifies the connection directed from the RATs to the RNS, while the term “downlink” identifies the connection directed from the RNS to the RATs.
The TDM frame is typically more efficient for handling full-duplex terminals while the TDMA portion provides a better handling of half-duplex terminals. The downlink TDMA mechanism is more complex to handle as it implies discontinuous reception at the RAT side, which means a more complex receiver implementation and higher costs for the RAT.
A downside of this approach is that it requires quite sophisticated circuitry on the radio access terminals.